Robotic arm to help decommission UK nuclear sites

Written by: Justin Cunningham | Published:

During the 1950s and 1960s when many nuclear power stations were built, engineers worked with an imperative of getting it done. The speed of technology development was the driving force and any thought of potential difficulty when it came to disposal was put to one side.

It must be said, this was not irresponsible engineering but was a response to the needs of the time, where thoughts and considerations of future clean up were way down the list of priorities. The result is that these once cutting edge technological marvels are in dire need of disposal, after being exposed to decades of high dose radiation.

To make matters all the more challenging, records and drawings of what exactly makes up the internals of many nuclear power stations can be sketchy, with some detail being lost to time. It means that decommissioning on some sites literally enters the unknown, with engineers unsure of what they might find.

However, decades on from the first round of nuclear builds and decommissioning, re-commissioning and redevelopment work is ramping up.
The first new build in more than 20 years was announced last month at Hinkley Point in Somerset, with more sites expected to be announced soon.

The imperative for the nuclear sector has of course changed since the 1950s and today engineers work toward the safe removal of contaminated hardware during decommissioning. Here, modern technology is playing a hand by taking human beings out of the loop and removing any risk to personnel.

Earlier this year, James Fisher Nuclear delivered a manipulator arm, dubbed Moduman 100, to a test facility in West Cumbria. The Energy Act 2004 saw the Government set up the Nuclear Decommissioning Authority (NDA) to deal specifically with the nuclear legacy and manage the decommissioning process.

It became clear that some elements would be better served using robotics so James Fisher Nuclear (JFN), which specialise in engineering equipment for challenging environments, designed and delivered a remote handling system to assist in the decommissioning process.

"We have a history of developing remote handling solutions for remote inspection, particularly for harsh environments, and with a focus on the nuclear industry," says Dr Carwyn Jones, systems data manager at James Fisher Nuclear. "There are large amounts of waste to process and that is challenging because of the age of the systems involved.

"A lot of the decommissioning is low level radiation, with restricted access however there is a need for a manipulator, to deal with that higher dose waste."

JFN wanted to design a manipulator arm that it could launch through standard through-wall tubes; a common method used to access areas where there is suspected high levels of radiation. These are usually between 250-270mm in diameter, which set in place an initial restraint for the team in terms of the manipulators arm diameter.

The difficulty and investment involved in making the Moduman, as well as its flexible operating nature, made JFN take the approach of developing it as a product that could be subsequently applied to other nuclear facilities in both UK and overseas. The flexibility of the system means it can also be mast mounted or attached to a remotely operated vehicle (ROV) for tasks such as future plant maintenance and inspection duties.

"The 100kg payload of the manipluator arm is for a tool rather than to carry out waste," says Dr Jones. "It has 6-degress of freedom and uses two 3-axis joysticks to control them. It is made up of six joints and an end effector, and is 2.7m long. The idea is it can go in to very awkward environments that are not just harsh, but humid, hot or acidic."

The manipulator uses a straightforward approach in its hydraulic drive system which is no doubt configured to give maximum reliability as it places vital components outside of its operating cell. The use of hydraulics also offers greater power density, an obvious advantage given the relatively tight envelope the arm has to operate within.

Each arm is driven by two hydraulic rams connected to a chain and sprocket that act in antagonistic pairs like muscles in the human arm, with the chain always under tension. Varying the pressure, and resulting tension, on each side determines how the arm moves.

"So it is very simple, with everything hydraulically driven being kept 'in-cell'," says Dr Jones. "We have two electric motors on the shoulder roll which are 'out-cell' but really that's it.

"One reason to use hydraulics is that it is simple and provides a high force, but it is different to a standard hydraulic system. The fluid itself is water soluble so it can be decommissioned much more easily than mineral oil, which is a very difficult to decommission using the general method of dilution, for obvious reasons."

The manipulator is also largely made from commercial off the shelf components including the hydraulic rams, the rotary joints that use standard HKS actuators, and the tool rotator which is driven by a Geroler motor.

Critical to the project was the motion control of the manipulator. Given the nature of its operational environment the team had to ensure repeatable motion of +/- 5mm accuracy at the end effector. To enable the motion control it used the LabVIEW Robotics platform module from National Instruments to configure the motion of the manipulator.

"We were able to easily program in the resolve motion, which means you drive and control the end effector, not the individual arms," says Dr Jones. "So when you move your hand you concentrate on your hand, but there are multiple muscles and joints enabling you to do that. And this is what we configured the manipulator to do, so you didn't have to worry about all the individual joins and actuators.

"LabVIEW is a deterministic system which is really important for our safety critical operation. And you can easily simulate the motion beforehand, which is obviously always good to make sure it works and to train operators."

The intuitive control system uses a Windows based touch screen and twin joysticks that can be desk mounted or the system can incorporate a portable rig linked to the arm via an umbilical cable. The manipulator can be operated in full joint control, world or tool modes if required, but resolve tip motion has been programmed and fitted as standard. Collision avoidance is another feature that proves handy given the environment as is its diagnosis function should there be a problem. The control system also features a training module that enables engineers on site to test the equipment's capability in a virtual environment modelled on the upcoming requirement.

The flexibility of the control system really is allowing engineers today cope with the challenges of nuclear decommissioning and answers calls of safe disposal at all time with minimal risk to those working on site.

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